DNA polymerases are a family of enzymes that use single-stranded DNA as a template to synthesize the complementary DNA strand. In particular, DNA polymerases can add free nucleotides to the 3′ end of a newly-forming strand resulting in elongation of the new strand in a 5′ to 3′ direction. Most DNA polymerases are multifunctional proteins that possess both polymerizing and exonucleolytic activities. For example, many DNA polymerases have 3′→5′ exonuclease activity. These polymerases can recognize an incorrectly incorporated nucleotide and the 3′→5′ exonuclease activity of the enzyme allows the incorrect nucleotide to be excised (this activity is known as proofreading). Following nucleotide excision, the polymerase can re-insert the correct nucleotide and replication can continue. Many DNA polymerases also have 5′→3′ exonuclease activity.
Sequencing by synthesis (SBS) determines the identity of the nucleotide sequence in a DNA strand. Polymerases used in SBS not only add nucleotides to the growing strand but also proofread the growing nucleotide strand. A 3′→5′ proofreading exonuclease domain is intrinsic to most DNA polymerases. As noted above, the exonuclease proofreading function can correct an incorrect base. In SBS, this exonuclease activity while sequencing will contribute towards deletion/insertion errors and is therefore undesirable. Standard mutations in the exonuclease domain to knock-out exonuclease activity in Family B polymerases typically reduce the strand displacement capability of the polymerase.
Therefore, there is a need for an exonuclease deficient polymerase that has an altered strand displacement characteristic relative to the standard mutations. Specifically, an exonuclease deficient polymerase with the same or improved strand displacement as a wild-type parental polymerase is desired. Improving strand displacement improves waiting time between consecutive nucleotide incorporations (i.e., decreases waiting time) as well as the extension/sequencing rate of the polymerase (i.e., increases the extension/sequencing rate).